Figure 10. A model of the origin of two IRBP genes in the teleosts

The presumptive ancestral gene in fish, predating the appearance of the teleosts is a gene much like the present day tetrapod gene with four Repeats encoded in a four-exon gene, with the first three repeats and part of the fourth found in the first exon. When the teleosts arose in Step 1, the whole genome was duplicated. In Step 2, the two IRBP genes drifted, deleting different parts of the gene. In the gene destined to become Gene 1, the 3' end of the gene was lost deleting the three introns, exons 2-4 and part of exon 1 (Step 2a), and the promoter underwent some alterations (Step 2b), so that it would be expressed in some ganglion cells and selected cells in the inner nuclear layer. The gene destined to become Gene 2 underwent an internal deletion, losing the parts of Exon 1 that encoded Repeats 2 and 3 (Step 2c) but Gene 2 did not undergo any significant changes in its promoter, retaining expression in photoreceptor cells and RPE cells. In Step 3, unequal crossing over between the two chromosomes would form the current teleost two-gene locus. Synteny is preserved in this model. GDF2 represents the growth differentiation factor 2 gene, and A8 represents the Annexin 8 gene.